add ch04 code along video (#573)

ch04/README.md

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 - [02_performance-analysis](02_performance-analysis) contains optional code analyzing the performance of the GPT model(s) implemented in the main chapter
 - [ch05/07_gpt_to_llama](../ch05/07_gpt_to_llama) contains a step-by-step guide for converting a GPT architecture implementation to Llama 3.2 and loads pretrained weights from Meta AI (it might be interesting to look at alternative architectures after completing chapter 4, but you can also save that for after reading chapter 5)
 
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+In the video below, I provide a code-along session that covers some of the chapter contents as supplementary material.
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+[![Link to the video](https://img.youtube.com/vi/YSAkgEarBGE/0.jpg)](https://www.youtube.com/watch?v=YSAkgEarBGE)